1/*- 2 * Copyright (c) 1985, 1993 3 * The Regents of the University of California. All rights reserved. 4 * 5 * Redistribution and use in source and binary forms, with or without 6 * modification, are permitted provided that the following conditions 7 * are met: 8 * 1. Redistributions of source code must retain the above copyright 9 * notice, this list of conditions and the following disclaimer. 10 * 2. Redistributions in binary form must reproduce the above copyright 11 * notice, this list of conditions and the following disclaimer in the 12 * documentation and/or other materials provided with the distribution. 13 * 3. All advertising materials mentioning features or use of this software 14 * must display the following acknowledgement: 15 * This product includes software developed by the University of 16 * California, Berkeley and its contributors. 17 * 4. Neither the name of the University nor the names of its contributors 18 * may be used to endorse or promote products derived from this software 19 * without specific prior written permission. 20 * 21 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 22 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 23 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 24 * ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 25 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 26 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 27 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 28 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 29 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 30 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 31 * SUCH DAMAGE. 32 */ 33 34#ifndef lint 35#if 0 36static char sccsid[] = "@(#)networkdelta.c 8.1 (Berkeley) 6/6/93"; 37#endif 38__attribute__((__used__)) 39static const char rcsid[] = 40 "$FreeBSD: src/usr.sbin/timed/timed/networkdelta.c,v 1.5 2007/11/07 10:53:41 kevlo Exp $"; 41#endif /* not lint */ 42#include <sys/cdefs.h> 43 44#include "globals.h" 45 46static long median(float, float *, long *, long *, unsigned int); 47 48/* 49 * Compute a corrected date. 50 * Compute the median of the reasonable differences. First compute 51 * the median of all authorized differences, and then compute the 52 * median of all differences that are reasonably close to the first 53 * median. 54 * 55 * This differs from the original BSD implementation, which looked for 56 * the largest group of machines with essentially the same date. 57 * That assumed that machines with bad clocks would be uniformly 58 * distributed. Unfortunately, in real life networks, the distribution 59 * of machines is not uniform among models of machines, and the 60 * distribution of errors in clocks tends to be quite consistent 61 * for a given model. In other words, all model VI Supre Servres 62 * from GoFast Inc. tend to have about the same error. 63 * The original BSD implementation would chose the clock of the 64 * most common model, and discard all others. 65 * 66 * Therefore, get best we can do is to try to average over all 67 * of the machines in the network, while discarding "obviously" 68 * bad values. 69 */ 70long 71networkdelta() 72{ 73 struct hosttbl *htp; 74 long med; 75 long lodelta, hidelta; 76 long logood, higood; 77 long x[NHOSTS]; 78 long *xp; 79 int numdelta; 80 float eps; 81 82 /* 83 * compute the median of the good values 84 */ 85 med = 0; 86 numdelta = 1; 87 xp = &x[0]; 88 *xp = 0; /* account for ourself */ 89 for (htp = self.l_fwd; htp != &self; htp = htp->l_fwd) { 90 if (htp->good 91 && htp->noanswer == 0 92 && htp->delta != HOSTDOWN) { 93 med += htp->delta; 94 numdelta++; 95 *++xp = htp->delta; 96 } 97 } 98 99 /* 100 * If we are the only trusted time keeper, then do not change our 101 * clock. There may be another time keeping service active. 102 */ 103 if (numdelta == 1) 104 return 0; 105 106 med /= numdelta; 107 eps = med - x[0]; 108 if (trace) 109 fprintf(fd, "median of %d values starting at %ld is about ", 110 numdelta, med); 111 med = median(med, &eps, &x[0], xp+1, VALID_RANGE); 112 113 /* 114 * compute the median of all values near the good median 115 */ 116 hidelta = med + GOOD_RANGE; 117 lodelta = med - GOOD_RANGE; 118 higood = med + VGOOD_RANGE; 119 logood = med - VGOOD_RANGE; 120 xp = &x[0]; 121 htp = &self; 122 do { 123 if (htp->noanswer == 0 124 && htp->delta >= lodelta 125 && htp->delta <= hidelta 126 && (htp->good 127 || (htp->delta >= logood 128 && htp->delta <= higood))) { 129 *xp++ = htp->delta; 130 } 131 } while (&self != (htp = htp->l_fwd)); 132 133 if (xp == &x[0]) { 134 if (trace) 135 fprintf(fd, "nothing close to median %ld\n", med); 136 return med; 137 } 138 139 if (xp == &x[1]) { 140 if (trace) 141 fprintf(fd, "only value near median is %ld\n", x[0]); 142 return x[0]; 143 } 144 145 if (trace) 146 fprintf(fd, "median of %ld values starting at %ld is ", 147 (intptr_t)xp-(intptr_t)&x[0], med); 148 return median(med, &eps, &x[0], xp, 1); 149} 150 151 152/* 153 * compute the median of an array of signed integers, using the idea 154 * in <<Numerical Recipes>>. 155 */ 156static long 157median(a, eps_ptr, x, xlim, gnuf) 158 float a; /* initial guess for the median */ 159 float *eps_ptr; /* spacing near the median */ 160 long *x, *xlim; /* the data */ 161 unsigned int gnuf; /* good enough estimate */ 162{ 163 long *xptr; 164 float ap = LONG_MAX; /* bounds on the median */ 165 float am = -LONG_MAX; 166 float aa; 167 int npts; /* # of points above & below guess */ 168 float xp; /* closet point above the guess */ 169 float xm; /* closet point below the guess */ 170 float eps; 171 float dum, sum, sumx; 172 int pass; 173#define AMP 1.5 /* smoothing constants */ 174#define AFAC 1.5 175 176 eps = *eps_ptr; 177 if (eps < 1.0) { 178 eps = -eps; 179 if (eps < 1.0) 180 eps = 1.0; 181 } 182 183 for (pass = 1; ; pass++) { /* loop over the data */ 184 sum = 0.0; 185 sumx = 0.0; 186 npts = 0; 187 xp = LONG_MAX; 188 xm = -LONG_MAX; 189 190 for (xptr = x; xptr != xlim; xptr++) { 191 float xx = *xptr; 192 193 dum = xx - a; 194 if (dum != 0.0) { /* avoid dividing by 0 */ 195 if (dum > 0.0) { 196 npts++; 197 if (xx < xp) 198 xp = xx; 199 } else { 200 npts--; 201 if (xx > xm) 202 xm = xx; 203 dum = -dum; 204 } 205 dum = 1.0/(eps + dum); 206 sum += dum; 207 sumx += xx * dum; 208 } 209 } 210 211 if (ap-am < gnuf || sum == 0) { 212 if (trace) 213 fprintf(fd, 214 "%ld in %d passes; early out balance=%d\n", 215 (long)a, pass, npts); 216 return a; /* guess was good enough */ 217 } 218 219 aa = (sumx/sum-a)*AMP; 220 if (npts >= 2) { /* guess was too low */ 221 am = a; 222 aa = xp + max(0.0, aa); 223 if (aa > ap) 224 aa = (a + ap)/2; 225 226 } else if (npts <= -2) { /* guess was two high */ 227 ap = a; 228 aa = xm + min(0.0, aa); 229 if (aa < am) 230 aa = (a + am)/2; 231 232 } else { 233 break; /* got it */ 234 } 235 236 if (a == aa) { 237 if (trace) 238 fprintf(fd, 239 "%ld in %d passes; force out balance=%d\n", 240 (long)a, pass, npts); 241 return a; 242 } 243 eps = AFAC*abs(aa - a); 244 *eps_ptr = eps; 245 a = aa; 246 } 247 248 if (((x - xlim) % 2) != 0) { /* even number of points? */ 249 if (npts == 0) /* yes, return an average */ 250 a = (xp+xm)/2; 251 else if (npts > 0) 252 a = (a+xp)/2; 253 else 254 a = (xm+a)/2; 255 256 } else if (npts != 0) { /* odd number of points */ 257 if (npts > 0) 258 a = xp; 259 else 260 a = xm; 261 } 262 263 if (trace) 264 fprintf(fd, "%ld in %d passes\n", (long)a, pass); 265 return a; 266} 267